Heat exchanger construction and thermal shield therefor



May 12, 1964 s. H. ESLEECK HEAT EXCHANGER CONSTRUCTION AND THERMALSHIELD THEREFOR 3 Sheets-Sheet 1 Filed Feb. 6, 1959 FIG. 1

SEE F|c.'3

SEE FIG. 5

INVENTOR. Samuel H. Esleeck ATTORNEY s. H. ESLEECK 3,132,691

HEAT EXCHANGER CONSTRUCTION AND THERMAL SHIELD THEREFOR 6, 1959 3Sheets-Sheet 2 FIG. 3

May 12, 1964 Filed Feb.

INVENTOR.

Samuel H. Esleeck ATTORNEY A CURVE A 1 AL. CURVE B "I FIG.4

CURVE C" INCHES A DEPTH FROM FACE 27 OF THERMAL SHIELD,

MEMBER DISPLACED ONE INCH CLOSER TO TUBE SHEET 14 0 o o O 6 5 4 3 4CURVE A THERMAL SHIELD EQUIDISTANT FROM THIN CURVE B THERMAL SHIELD WITHWEDGE SHAPED CHAMBER 25 CURVE C THERMAL SHIELD EQUIDISTANT FROM THINMEMBER May 12, 1964 s. H. ESL EECK 3,132,691

HEAT EXCHANGERCONSTRUCTION AND THERMAL SHIELD THEREFOR Filed Feb. 6,1959 3 Sheets-Sheet 5 INVENTOR.

Samuel H. Esleeck ATTORNEY United States Patent 3,132,691 HEAT EXCHANGERCONSTRUCTEUN AND THEM-EAL SHEELD THEREFQR Sam-nei H. Esieeek, Lynchhnrg,Va., assignor to The Bahcoclr a Wiicox Company, New York, N.Y., acorporation of New Jersey Filed Feb. 6, 195%, Ser. No. 791,677 4 Claims.(Cl. 165-134) This invention relates in general to heat exchangers andmore specifically, it relates to a new and improved means for reducinghigh thermal gradients in such heat exchangers at the juncture of thickand thin walled members therein.

In a pressure vessel construction wherein a thick and a thin member arejoined, such a joint becomes an area subject to stress concentrationsdue to the reaction of the thick and thin members to pressuredifferentials. When such a construction, in addition, is subjected tothermal gradients, the resulting thermal stresses are additive to theaforementioned pressure stresses and these cumulative stresses must beused to design the vessel.

The present invention provides a means for reducing the high thermalgradients thereby making such heat exchanger construction commerciallymore attractive.

The present invention describes means for forming a wedge shaped chamberof quiescent fluid adjacent to the juncture of the thick and thinmembers of a heat exchanger. Ihis wedge shaped chamber reduces the highthermal gradient in each increment of length by distributing the totaltemperature change over a greater length of the members.

This invention further provides means, in conjunction with the wedgeshaped chamber, to maintain within the chamber the same heat transferfluid as is present in the main body of fluid within the shell side ofthe heat exchanger.

Moreover, this invention furnishes means for maintaining the fluid inthe chamber in a quiescent state, thereby decreasing its coefficient ofthermal conductivity.

The invention also provides means, in conjunction with a tube in theheat exchanger, for forming a wedge shaped annular chamber around thetube with its apex at the.

juncture of the tube and the tube sheet.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which I have illustrated and described a preferred embodimentof the invention.

Of the drawings:

FIG. 1 is a longitudinal section of a shell and tube heat exchangerillustrating the present invention;

FIG. 2 is a cross section of the heat exchanger taken along the line 22of FIG. 1 with a portion broken away;

FIG. 3 is an enlarged section at the juncture of the tube sheet andshell as generally indicated in FIG. 1;

3,132,691 Patented May 12, 1964 "Ice - adjacent each tube sheet 14- and15' so as to provide a space to maintain a layer of quiescent fluid 28between the thermal shield and the tube sheet (see FIGS. 3 and 5) and alayer of quiescent fluid 29 between the thermal shield and the shell(FIG. 3). This thermal shield surrounds each tube and has a means suchas surfaces 37 and 24 adjacent the tubes and the shell to form, inconjunction with the. inner surface of the shell, and the outer surfaceof the tube, wedge shaped annular chambers 25 and 38. These wedge shapedchambers 25 and 38 are further defined by a baflle ring 26 secured tothe thermal shield 23 at its outer surface 27, the ring 26 helping tomaintain the fluid in chambers 25 and 38 in a quiescent state.

For a further clarification of the invention, a description of theoperation of the heat exchanger of FIG. 1 follows. A hot heat transferfluid flows through the shell 11 and across tubes 16 via inlet andoutlet nozzles 21 and 22 respectively, releasing heat by indirect heattransfer relationship to a cooler fluid flowing through the tubes 16,the cooler fluid flowing serially through the inlet'nozzle 19, the inletchamber 17, the tubes 16, the outlet chamber 18, and the outlet nozzleZtl.

As an example of a specific embodiment of the present invention, thefollowing design is given. In the heat exchanger of FIG. 1, thecomponents, including the thermal shield, are fabricated of austeniticstainless steel. At the location where the shell 11 joins the tube sheetand hemispherical head, the shell has a thickened section as indicatedby 11A. Referring to FIG. 3, the thermal shield, in association with thetube sheet 14 forms a gap 28, inch wide, and in association with thethickened part of the shell 1 1A a gap 29, /8 inch wide both of whichserve as a repository for the quiescent fluid. The wedge shaped chamber25 in the thermal shield is circular in section, having a diameter atsurface 27 of 1% inches less than the inside diameter of the shell 11,and with a uniform tapered surface 24 such that, at a distance of 2 /2inchesfrom the surface 27, the diameter of the thermal shield 23 is but/8 inch less than the inside 1 diameter of the shell 11 thereby formingan included FIG. 4 is a graph comparing the thermal gradient with threemodifications of a thermal shield, including the present invention,drawn to the same scale and in relation with the section shown in FIG.3;

'FIG. 5 is an enlarged section of the present invention at the junctureof the tube and tube sheet as generally angle of 12% degrees between theshell and the surface 24. Thereafter, the thermal shield forms aconstant gap 29 adjacent to the thickened section 11A of the shell. TheA3 inch thick baflie ring 26, in this example, restricts the opening tothe chamber 25 by leaving a inch clearance with the shell 11.

Referring to FIG. 5, the thermal shield 23, in association with the tubesheet 14, forms a continuation of the gap 28 which is A inch wide andserves the same function as described above. The wedge shaped chamber 38formed in the thermal shield has an opening, circular in section, 1%inches larger in diameter than the outside diameter of the tube 16 andarranged symmetrically thereabout at the surface 27 of the thermalshield 23. Following the conical surface 37, the circular sectionreduces in diameter to only A; inch larger in diameter than the tube 16,at the gap 28. The baflie ring 26 is for restricting the opening to thechamber 38 and is concentric to each tube 16.

In this specific illustration, liquid sodium is the hot heat transferfluid flowing outside thetubes entering through nozzle 21 and leavingthrough nozzle 22. Pressurized water is the cooler fluid enteringthrough nozzle 19 then flowing through the tubes 16, thence throughoutlet nozzle 20.

While specific examples of materials, fluids, and critical dimensionshave been given, it is to be understood that other materials, fluids anddimensions may be employed in producing the invention; for instance,other alloys of steel, carbon steel, or non-ferrous metals may be used.Also other fluid combinations may be used, such as Water to water, waterto steam, steam to steam, liquid metal to stearrn, and organic fluid towater, organic fluid to organic fluid, etc. The liquid metal used wouldnot be limited to liquid sodium, but may be any liquid metal with thedesiuable thermd properties.

In considering a heat exchanger constructed and operated as describedabove but Without the thermal shield, the thick tube sheet has thehotter fluid flowing adjacent to one face thereof, i.e. the tube side,and the cooler fluid flowing adjacent the opposite face. The hotterfluid causes the face with which it is in contact to expand to an extentdictated by the temperature of the hotter fluid. The cooler face of thetube sheet tends to expand to the extent dictated by the temperature ofthe cooler fluid flowing on that side, thereby causing diflerent degreesof expansion in a single piece of material. These expansiondifferentials pnoduce diiferential forces which act on the two faces,setting up a stress within this member, this stress being commonlyreferred to as the thermal stress. In the case of a thin member beingjoined to a thick member, as a tube joined to a tube sheet, or thethinner section of the shell joined to the thicker section, thesethermal stresses are accentuated due to the further difference ofthermal expansion of the thin member simultaneously in contact with botha hot fluid and a cold fluid and the mutual restraining forces exertedby each member upon the other.

The thermal shield 23 as illustrated in FIGS. 1, 2, 3 and 5 is notfixedly attached to either the thick or thin members and thus it is freeto expand in all directions thereby transmitting no thermally causedstresses to these members. This shield is made of material having a lowthermal conductivity as compared to the heat transfer fluid in which itis located. The quiescent layer of fluid between the thermal shield 23and the tube sheets 14 or 15 or the shell 11 and 11A in combination withthe low thermal conductivity of the thermal shield, as compared to thatof the fluid, limits the heat flow from the hot fluid into the tubesheet or shell section, thereby reducing the thermal stresses imposedthereon. However, if the thermal shield were everywhere equidistant fromthe tube 16, or the shell 11 as illustrated by the dotted lines 36 and39 in FIGS. 3 and 5, the thermal gradient in the thin member would notbe reduced in degree but would only be displaced along the thin memberaway from the thick member as illustrated by curves A and C in FIG. 4.In this instance, curve C illustrates the thermal gradient with athermal shield 3" thick and curve A illustrates the thermal gradientwith a thermal shield 4" thick as shown in FIG. 3. If this thermalgradient were only displaced along the shell away from the tube sheetthe high gradient would occur at the juncture of the thick and thinsections of the shell, and could make the high stress imposed upon thispoint prohibitive from a design standpoint.

Without the present invention, but using a thermal shield having aboundary as indicated by line 36 (or 39), this thermal gradient occursin the thin member due to the fact that while it it not mechanicallyattached to the thermal shield, it is in good thermal contact therewithdue to the heat transfer fluid between them. This fluid gap offers verylittle resistance to heat flow as compared to the thermal shield. Asheat flows into the face 27 of the thermal shield in contact with thehot fluid, it meets with an increasing amount of thermal resistance inthe form of the thermal shield itself. Since a flow of heat through asolid is analogous to the flow of current through an electrical circuit,it may be understood that the heat flow will seek the path of leastresistance. Thus, a major portion of the heat entering the face 27 ofthe thermal shield will soon cross the small fluid gap adjacent surface36 and enter the thin member. In essence, the thermal energy enters thethermal shield in an axial direction, but soon the major portion isdiverted in a radial direction towards the thin member. The rate of heatinput into the thin member is then quickly reduced because of the highresistance of the thermal shield to heat flow. Temperature gradients area function of the change in thermal flow; therefore, if the thermal flowis quickly reduced, a high temperature gradient will result asillustrated in FIG. 4, curve A, where the temperature change from O to 1inch in depth is the major portion of the temperature dif ferential from0 to 4 inches in depth.

By beveling the thermal shield adjacent to the thin member, i.e. thetube or thin part of the shell, as taught by this invention, a taperedgap 25 exists between the thermal shield and the thin member. The volumeof this wedge shaped chamber, formerly filled with a material of highthermal resistivity, is now filled with a material of relatively highthermal conductivity, i.e. the heat transfer fluid. The high radial heatflow described above still exists but now an appreciable axial flow ofheat occurs in the fluid wedge which gradually decreases as the volumeof the fluid chamber decreases. This axial heat flow pattern reduces thetemperature gradient in the gap along the axial direction and thusreduces the gradient along the thin member as compared to the gradientexisting in the former example.

In FIG. 4, curves B and A illustrate the difference in the thermalgradient in the thin walled member with the present invention of a wedgeshaped chamber 25 as compared to the use of a thermal shield 23 withoutthe wedge shaped chamber and using a thermal shield shaped as shown bythe dotted 'line 36. In the specific heat exchanger example given above,with the liquid metal at 670 F. and the pressurized water at 297 F., atemperature drop of 334 took place in the shell in a distance of oneinch from the face of the thermal shield as shown in curve A. With theuse of the present invention, this same temperature differential wasmodulated so it occurred over a distance of 2 /2 inches from the face ofthe thermal shield as shown in curve B.

Where a thermal shield is not required to protect a tube sheet, thethermal gradient in a tube may be reduced by the alternate arrangementas illustrated. in FIG. 6 which shows a section through a tube sheet 30in which a tube 31 is firmly seated. The inner face 32 of the tube sheet30 is provided with a conical surface 33 concentric to tube 31 andextending approximately one-half the depth of the tube sheet from theface 32. This conical surface 33, in conjunction with the tube 31, formsan annular wedge shaped chamber 34 around the tube which serves toreduce the thermal gradient present at the juncture of the tube sheetand tube. This wedge shaped chamber operates in the same manner asdescribed above and may be provided with a bafiie ring 35 to keep thefluid in the chamber 34 in a quiescent state.

While in accordance with the provisions of the statutes I haveillustrated and described herein the best form and mode of operation ofthe invention now known to me, those skilled in the art will understandthat changes may be made in the form of the apparatus disclosed withoutdeparting from the spirit of the invention covered by my claims, andthat certain features of my invention may sometimes be used toadvantagewithout a corresponding use of other features.

I claim:

1. A heat exchanger comprising a tube sheet having opposed first andsecond faces, a tube extending through the tube sheet and contacting thesecond face of the tube sheet, means for passing a cooling fluid incontact with the first face of the tube sheet and through the tube,means for passing a heating fluid having a higher temperature than thecooling fluid in contact with the outer surface of the tube and thesecond face of the tube sheet causing heat to flow through the tube tothe tube sheet resulting in a thermal gradient along the tube, meansforming a frusto-conically shaped fluid chamber surrounding the portionof the tube subject to said thermal gradient and including an outer facein contact with the heating fluid, the small end of said chamber locatedat said point of contact of the tube and the tube sheet and the largeend of said chamber located at said outer face, said chamber being incommunication with and occupied by said heating fluid, and a bafliemeans associated with said chamber forming means and said tube locatedat the large end of said chamber to substantially close the large end ofsaid fluid chamber to maintain said fluid therein in a substantiallyquiescent state to reduce said thermal gradient along said tube.

2. A heat exchanger comprising a fluid-tight container having at leastone tube sheet having opposed first and second faces, a tube seat formedthrough said tube sheet, a tube extending Within said container andhaving an end thereof fitted into said tube seat and securely fastenedto said tube sheet, means for passing a first heat transfer fluid incontact with the first face of the tube sheet and through said tube,means for passing a second heat transfer fluid at a temperaturedifferent than that of said first fluid through said container incontact with the second face of said tube sheet and about said tubecausing heat to flow between said tube and said tube sheet resulting ina thermal gradient along said tube, means forming a frusto-conicallyshaped fluid chamber in said second-face of said tube sheet surroundingsaid tube, the small end of said chamber located at said point ofcontact between said tube and said tube sheet and the large end locatedat the second'face of the tube sheet, said chamber being incommunication with and occupied by said second heat transfer fluid, anda baflie ring surrounding said tube at the large end of saidwedge-shaped chamber and substantially closing the end of said chamberto maintain the fluid therein in a substantially quiescent state toreduce the thermal gradient along said tube.

3. A heat exchanger comprising a fluid-tight container having at leastone tube sheet having opposed first and second faces, a tube seat formedthrough said tube sheet, a tube extending within said container andhaving an end thereof fitted into said tube seat and securely fastenedto said tube sheet, means for passing a first heat transfer fluid incontact with the first face of said tube sheet and through said tube,means for passing a second heat transfer fluid at a temperaturedifferent than that of said first fluid through said container incontact with the second face of said tube sheet and about said tubecausing heat to flow between said tube and said tube sheet resulting ina thermal gradient along said tube, a thermal block disposed around saidtube and having a first face adjacent said second face of said tubesheet and an oppositely disposed second face, means forming afrusto-conically shaped fluid chamber in said thermal block around andcontiguous to the outer surface of said tube, said chamber having thesmall end located at the first face of said thermal block and its largeend at the second face of said thermal block, said chamber being incommunication with and occupied by said second heat transfer fluid, anda baflie ring surrounding said tube at the widest portion of saidwedge-shaped chamber and substantially closing the end of said chamberto maintain the fluid therein in a substantially quiescent state toreduce said thermal gradient along said tube.

4. A heat exchanger comprising a fluid-tight container having at leastone tube sheet having opposed first and second faces, a tube seat formedthrough said tube sheet, a tube extending within said container andhaving an end thereof fitted into said tube seat and securely fastenedto said tube sheet, means for passing a first heat transfer fluid incontact with the first face of said tube sheet and through said tube,means for passing a second heat transfer fluid at a temperaturedifferent than that of said first fluid through said container incontact with the second face of said tube sheet and about said tubecausing heat to flow between said tube and said tube sheet resulting ina thermal gradient along said tube, a thermal block disposed around saidtube and having a first face adjacent said second face of said tubesheet and an oppositely disposed second face, means forming afrusto-conically shaped fluid chamber in said thermal block around andcontiguous to the outer surface of said tube, said chamber having thesmall end located at the first face of said thermal block and its largeend at the second face of said thermal block, said chamber being incommunication with and occupied by said second heat transfer fluid, anda baflie ring surrounding said tube at the widest portion of saidwedge-shaped chamber and substantially closing the end of said chamberto maintain the fluid therein in a substantially quiescent state toreduce said thermal gradient along said tube, said thermal block beingimmersed in said second heat transfer fluid' and having a lower thermalconductivity than said fluid in which it is immersed.

References Cited in the file of this patent UNITED STATES PATENTS

1. A HEAT EXCHANGER COMPRISING A TUBE SHEET HAVING OPPOSED FIRST ANDSECOND FACES, A TUBE EXTENDING THROUGH THE TUBE SHEET AND CONTACTING THESECOND FACE OF THE TUBE SHEET, MEANS FOR PASSING A COOLING FLUID INCONTACT WITH THE FIRST FACE OF THE TUBE SHEET AND THROUGH THE TUBE,MEANS FOR PASSING A HEATING FLUID HAVING A HIGHER TEMPERATURE THAN THECOOLING FLUID IN CONTACT WITH THE OUTER SURFACE OF THE TUBE AND THESECOND FACE OF THE TUBE CAUSING HEAT TO FLOW THROUGH THE TUBE TO THETUBE SHEET RESULTING IN A THERMAL GRADIENT ALONG THE TUBE, MEANS FORMINGA FRUSTO-CONICALLY SHAPED FLUID CHAMBER SURROUNDING THE PORTION OF THETUBE SUBJECT TO SAID THERMAL GRADIENT AND INCLUDING AN OUTER FACE INCONTACT WITH THE HEATING FLUID, THE SMALL END OF SAID CHAMBER LOCATED ATSAID POINT OF CONTACT OF THE TUBE AND THE TUBE SHEET AND THE LARGE END